Systems that merge video signals and/or graphics are conventionally very memory intensive. In order to combine video signals into a single output data stream synchronized with an output device, such systems implement the following functions:
(1) Demodulate incoming composite video signals into their constituent components;
(2) Synchronize the demodulated signals with the output device via a memory element;
(3) Merge or multiplex the synchronized signals together to form an output data stream which is sent to the output device.
In most applications there will be a master signal that is already synchronized with the output device. In the case of a television receiver the master signal would be the main `picture`. In the case of a computer type display the master signal might be the data stream from the graphics layer.
In the article `IC Set for a Picture-in-Picture System with On-Chip Memory`, IEEE Transactions on Consumer Electronics, Vol 36, No. 1, (Feb 1990), pp 23-31, Mr Burkert et al discuss a picture-in-picture system consisting of two devices, namely an "Analog Digital Converter Interface for the Inserted Picture" (ADIIP) and a "Picture Insertion Processor" (PIP). The ADIIP adjusts an analog video channel for the inserted picture to the digital inputs of the PIP and requires the following four analog input signals from a standard color decoder:
Y--A Luminance signal; PA1 U,V--The Chrominance signals; and PA1 Sand--A pulse used as a horizontal timing reference. PA1 Multimedia Workstations PA1 Video Conferencing Devices (Analogue Frequency Domain Multiplex Transmission) PA1 `Picture in Picture` Television Receivers PA1 `Picture in Picture` Video Tape Recorders PA1 General Composite Video Capture Devices
A digital data stream is produced by the ADIIP and sent to the PIP, where it is passed through decimation filters, the resultant data being stored in an internal memory. This data is read out of the memory at a rate that is synchronized with the external line frequency of the master (parent) channel, and is output from the PIP as analog output signals having the component signals Y, -(B-Y), -(R-Y). Alternatively the data from the memory can firstly be converted into RGB signals which are then converted to analog and output from the PIP.
The article `Development of a Digital TV system for use in Computer Systems`, by K Kohiyama et al, IEEE Transactions on Consumer Electronics, Vol. 35, No.3, (Aug 1989), pp 624-629, describes a system for merging and integrating computer and TV images. In this system a non-standard digital TV signal is divided into a color signal and an intensity signal by a Y/C separator circuit. These non-standard signals are then converted into standard TV signals in an AFC circuit which detects the phase difference between the color burst signal and the horizontal sync signal in the non-standard signal and then performs a linear interpolation operation on this signal according to the detected phase difference to produce a standard TV signal. The standard TV signals are then adapted for storage, converted to RGB signals and stored in a digital memory. These RGB signals can then be read from memory at any desired rate. For example they can be read out at a computer scan rate, converted back to analog signals, and then superimposed on a computer generated image.
The article `Picture in Picture System with Digital Memory for VCRS`, by M Masuda et al, IEEE Transactions on Consumer Electronics, Vol. CE33, No. 3, (Aug 1987), pp 230-239, discloses a Picture-in-Picture system for VCRs that uses a dual port memory. A VCR has two inputs, namely a signal from the tuner and a playback signal. The system allows both signals to be viewed simultaneously, one signal forming the main picture and the other signal forming a superimposed sub-picture. The system performs processing steps on the component signals of the sub-picture, time compresses the sub-picture, and combines the main picture with the sub-picture for display on a single TV screen. In this process the input sub-picture is separated into a luminance signal and a color difference signal and these analog signals are then converted to 6-bit digital signals before being stored in the dual port memory. These signals are then read out at a different rate, converted back into analog signals, and combined with the main signal for display.
European patent application EP 0,288,152 describes a video signal generator for generating a signal representing both a main picture and an auxiliary picture simultaneously. An auxiliary video signal demodulated into chrominance and luminance values is sampled and samples from every third line of the auxiliary video signal are stored in a self-sequencing memory as part of a compressed auxiliary signal. A sync separator also receives the auxiliary video signal in order to generate a clocking signal to determine when the sampled signal is written into memory, whilst a write address generator creates write address information to determine where the sampled signal is written to within the memory. The compressed auxiliary signal information is then read from memory at a rate and from memory locations determined by the demodulated main video signal so as to be synchronous with the main video signal. This compressed signal information is then substituted for appropriate main signal sample information to create combined samples which are sent to a `Pix-in-Pix` video signal processor.
All of the above mentioned systems require a large amount of memory space in order to store the component signal data of the demodulated video signal. This memory space has been reduced in these prior art systems to some extent by performing various packing functions on the video data before it is stored in memory. However such packing processes are undesirable since they cause degradation in the signal to noise ratio (SNR) and aliasing artefacts to be introduced. Generally the greater the packing, the greater the degradation of the signal, but the smaller the amount of memory required. Hence a compromise is necessary to achieve an acceptable result. A typical packing standard which is regarded as acceptable for consumer use is CCIR 601.